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The government-ordered switch to digital television broadcasting next year promises razor-sharp picture and orchestra-like sound -- that is, if the signal actually comes in.

Jennifer Jackson, a 26-year-old medical student who lives in Arlington, is discovering that despite her efforts, it might not. She has a digital converter box, the receiver of choice for those who rely on an antenna and do not have a digital television. But she is finding that the new digital signals are more capricious than old-fashioned analog.

The picture's clarity is impressive, she said, until an airplane flies by on its way to nearby Reagan National Airport -- a frequent event in her 20th-floor apartment. Airplane traffic used to cause seconds-long bouts of fuzziness in the analog picture. But digital signals are more sensitive to disruption, so the sound mutes and the screen freezes, sometimes dissolving into a cascade of pixels. Similar glitches happen when her roommate walks through certain parts of the living room.

"The picture's great when I get good reception," said Jackson, who had wrapped her "rabbit-ear" antenna in aluminum foil to boost analog reception, "but it can be annoying when you're trying to pay attention to a show and the picture keeps falling apart."

The nation's broadcasters will shut off traditional, over-the-air TV signals as they move to all-digital programming Feb. 17. But questions remain about whether the digital signals will consistently reach the 14 million households that depend on antennas to receive broadcasts.

A large number of viewers who hook up a converter box, or use a newer digital TV, will have clearer reception and more channels than they did with analog broadcasts. For some, the improved quality could serve as a replacement for more-expensive basic cable service. But some consumers may have to buy extra equipment to reliably pull in as many channels as they did before the switch, especially if they live in rural areas or near tall trees or buildings.

"The government's message for consumers is that all they need is a converter box or digital TV," said David Klein, executive vice president of Centris, a market research firm. "That's an oversimplification of what's going to happen."

In the Washington area, about 56 percent of the 370,000 households watching over-the-air broadcasts may need to upgrade their antennas, according to Centris. Nationwide, Centris estimates that about half such households will need a new antenna.

Most broadcasters are already airing digital broadcasts along with analog programming, but some digital antennas are not in their final positions on transmission towers. In February, when digital antennas replace analog equipment -- by being moved from the side of the tower to the top, for example -- reception should improve, according to the Federal Communications Commission.

Locally, many major network stations said they were already broadcasting at full digital power and have moved their digital antenna to the top of their towers, so viewers should not notice major reception changes in February. Most of the local towers are in Northwest Washington.

People using antennas to get signals on older analog TV sets will need a converter box, which costs $40 to $80, to keep watching TV, and they can apply for a $40 government-sponsored coupon to offset the cost. Digital TVs will receive the digital broadcasts but will still need an antenna's help. Cable and satellite subscribers who receive digital programming should not have to do anything for the transition.

What new digital audiences have to fear is the "digital cliff," or the all-or-nothing quality of digital reception. The picture is excellent until the signal weakens or is interrupted, causing the picture to disappear, and it is more sensitive to interference from hills, trees, buildings and bad weather than traditional analog reception. An analog picture degrades gradually, getting more static and snow as signals weaken.